Photoluminescence of ZrO2:Gd3+ and ZrO2:Dy3+ coatings formed by the plasma electrolytic oxidation

Zirconia (ZrO2) coatings, doped with Gd3+ and Dy3+ in various concentrations, were created by the plasma electrolytic oxidation (PEO) process of zirconium by the addition of Gd2O3 and Dy2O3 particles to the supporting electrolyte, respectively. Their morphological, chemical, phase, and above all, photoluminescent (PL) properties were examined. Morphologies resemble the typical appearance for PEO created coatings. Incorporated rare-earth species are uniformly distributed over the coatings. Gd3+ ion gets more easily incorporated than Dy3+ by the PEO. The main identified phase of ZrO2 is monoclinic. Gd3+ induces the morphing to the tetragonal phase, proportional to its doping concentration. Gd3+ exhibits maximum emission at 313 nm, corresponding to the 6P7/2 → 8S7/2 transition, by the 278 nm excitation, with intensity proportional to the doping concentration. The spectra also feature a broad-band originating from ZrO2 host matrix. The ZrO2:Dy3+ spectra feature peaks at typical positions for 4f-4f transitions of Dy3+, overlapping with ZrO2 PL. The appearance of new sharp peaks when exciting into the charge transfer band of Dy3+-O2- in comparison to the direct 4f-4f excitation is due to the emissions from the higher levels of Dy3+. PL emission intensity of ZrO2 drops with increasing Gd3+ or Dy3+ concentrations, indicating an efficient energy transfer mechanism. CIE diagram showed that the emissions of ZrO2:Dy3+ correspond to the cool-white (at ca. 4000 K) and warm-white (at ca. 3000 K) light, by the 355 nm and 280 nm excitations, respectively